RDMA/i40iw: Handle workqueue allocation failure

alloc_ordered_workqueue may fail and return NULL.  The fix captures the
failure and handles it properly to avoid potential NULL pointer
dereferences.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Shiraz, Saleem <shiraz.saleem@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>